diff --git a/mingw-w64-crt/testcases/t_mbrlen.c b/mingw-w64-crt/testcases/t_mbrlen.c
index e169107..4b1c46c 100644
--- a/mingw-w64-crt/testcases/t_mbrlen.c
+++ b/mingw-w64-crt/testcases/t_mbrlen.c
@@ -9,6 +9,16 @@
 #include <stdlib.h>
 #include <wchar.h>
 
+/**
+ * This test is for mingw-w64's implementation of mbrlen function.
+ *
+ * This implementation is used with all msvcr*.dll CRTs, but not UCRT.
+ * This test is skipped for UCRT.
+ *
+ * Also note that mingw-w64's implementation only works with SBCS and DBCS
+ * code pages (MB_CUR_MAX == 1 || MB_CUR_MAX == 2).
+ */
+
 static void set_conversion_state (mbstate_t *state, int bytes) {
 #ifdef _UCRT
   state->_Wchar = bytes;
diff --git a/mingw-w64-crt/testcases/t_mbrtowc.c b/mingw-w64-crt/testcases/t_mbrtowc.c
index c33cc22..4d97b90 100644
--- a/mingw-w64-crt/testcases/t_mbrtowc.c
+++ b/mingw-w64-crt/testcases/t_mbrtowc.c
@@ -9,6 +9,16 @@
 #include <stdlib.h>
 #include <wchar.h>
 
+/**
+ * This test is for mingw-w64's implementation of mbrtowc function.
+ *
+ * This implementation is used with all msvcr*.dll CRTs, but not UCRT.
+ * This test is skipped for UCRT.
+ *
+ * Also note that mingw-w64's implementation only works with SBCS and DBCS
+ * code pages (MB_CUR_MAX == 1 || MB_CUR_MAX == 2).
+ */
+
 static void set_conversion_state (mbstate_t *state, int bytes) {
 #ifdef _UCRT
   state->_Wchar = bytes;
diff --git a/mingw-w64-crt/testcases/t_mbsrtowcs.c b/mingw-w64-crt/testcases/t_mbsrtowcs.c
index e85719e..e9f60ec 100644
--- a/mingw-w64-crt/testcases/t_mbsrtowcs.c
+++ b/mingw-w64-crt/testcases/t_mbsrtowcs.c
@@ -9,6 +9,16 @@
 #include <stdlib.h>
 #include <wchar.h>
 
+/**
+ * This test is for mingw-w64's implementation of mbsrtowcs function.
+ *
+ * This implementation is used with all msvcr*.dll CRTs, but not UCRT.
+ * This test is skipped for UCRT.
+ *
+ * Also note that mingw-w64's implementation only works with SBCS and DBCS
+ * code pages (MB_CUR_MAX == 1 || MB_CUR_MAX == 2).
+ */
+
 /* ASCII text */
 char          AsciiText[] = "Simple English string.";
 /* SBCS text (code page 1252) */
diff --git a/mingw-w64-crt/testcases/t_wcrtomb.c b/mingw-w64-crt/testcases/t_wcrtomb.c
index 99329f2..f9d1a8d 100644
--- a/mingw-w64-crt/testcases/t_wcrtomb.c
+++ b/mingw-w64-crt/testcases/t_wcrtomb.c
@@ -9,6 +9,16 @@
 #include <stdlib.h>
 #include <wchar.h>
 
+/**
+ * This test is for mingw-w64's implementation of wcrtomb function.
+ *
+ * This implementation is used with all msvcr*.dll CRTs, but not UCRT.
+ * This test is skipped for UCRT.
+ *
+ * Also note that mingw-w64's implementation only works with SBCS and DBCS
+ * code pages (MB_CUR_MAX == 1 || MB_CUR_MAX == 2).
+ */
+
 #define WIN32_LEAN_AND_MEAN
 #include <windows.h>
 
diff --git a/mingw-w64-crt/testcases/t_wcsrtombs.c b/mingw-w64-crt/testcases/t_wcsrtombs.c
index 2a78c78..705cdc2 100644
--- a/mingw-w64-crt/testcases/t_wcsrtombs.c
+++ b/mingw-w64-crt/testcases/t_wcsrtombs.c
@@ -10,6 +10,16 @@
 #include <string.h>
 #include <wchar.h>
 
+/**
+ * This test is for mingw-w64's implementation of wcsrtombs function.
+ *
+ * This implementation is used with all msvcr*.dll CRTs, but not UCRT.
+ * This test is skipped for UCRT.
+ *
+ * Also note that mingw-w64's implementation only works with SBCS and DBCS
+ * code pages (MB_CUR_MAX == 1 || MB_CUR_MAX == 2).
+ */
+
 wchar_t AsciiText[] = L"Simple English text.";
 wchar_t SBCSText[] = L"Sömè fÛnnÿ têxt";
 wchar_t DBCSText[] = L"日本語テクスト";